The apparatus includes a conductor structure with at least one signal path and two reference paths arranged symmetrically to the signal path. The signal path and the two reference paths together form a coplanar line. The conductor structure is arranged on two oppositely lying sides of at least one dielectric substrate layer of a predetermined thickness in such a manner that the conductor structure overlaps in predetermined coupling regions, whereby the coupling regions of the conductor structure transfer the high-frequency signals by an electromagnetic coupling.
This type of galvanic isolation is found, for example, in measuring devices of process measurements technology. Such measuring devices are used frequently, in automation- and process-control-technology, for measuring a process variable, such as e.g. flow, fill level, pressure, temperature or some other physical and/or chemical process variable in the course of a process. For example, Endress+Hauser produces and sells measuring devices under the mark MICROPILOT®, which work according to the travel-time measuring method and serve for ascertaining and/or monitoring fill level of a medium in a container. In the travel-time measuring method, for example, microwaves, respectively radar waves, or ultrasonic waves, are emitted via an antenna, and the echo waves reflected on the surface of the medium are received, following the distance-dependent travel time of the signal. From the travel-time, the fill level of the medium in a container can then be calculated. A further measuring principle, of a multitude of measuring methods for ascertaining the fill level in a container, is that of guided microwaves, respectively the TDR (Time Domain Reflection) measuring method. In the TDR measuring method, e.g. a high-frequency pulse is generated along a Sommerfeld surface waveguide or coaxial waveguide, and such is then partially reflected back, upon encountering a jump in the DC (dielectric constant) value of the medium surrounding the surface waveguide. From the time difference between the emission of the high-frequency pulse and receipt of the reflected echo signal, the fill level can be ascertained. The so-called FMCW (Frequency Modulated Continuous Waves) method is, likewise, performable in connection with the above principles of measurement.
The type of apparatus to which the present invention relates is used for assuring a galvanic isolation between an earth-grounded process space and a measuring device. Galvanic isolation is needed in process measurement technology, since the process space, or the elements in contact with the process, must be at earth-potential, due to the requirements of protection against explosion. The reference grounds of the measuring devices, however, mostly deviate from earth-potential. The difference between the two potentials results in a voltage lying between the earth-grounded process elements and the measuring device, whereby a current is generated. This current has the disadvantage that the lines of a reference ground are strongly loaded by current flow. This has the effect, that the temperature of a ground line can strongly increase, so that the ignition protection category “Intrinsically Safe” of the measuring device can no longer be assured.
In the case of commercial measuring devices, isolation of the electrical current circuit is mostly effected in the input region of the measuring device, i.e. the current-feeding lines and the signal lines are galvanically isolated. The galvanic isolation of the current-feeding lines is, in the case of an applied, alternating current, mostly effected via an inductive coupling by means of a transformer or via a capacitive coupling by means of capacitors. In the case of a direct-current supply of the measuring device, a direct-current converter separates the supply lines of the measuring device, or the current flowing in the lines is limited via supplementary components. Galvanic isolation of the data line, as regards the signal, is mostly effected via an optocoupler. Altogether, this form of embodiment of explosion protection in the input module of the measuring device has the disadvantage that a multitude of expensive and disturbance-susceptible components is required in connection with this galvanic isolation of the elements located in the process space from the equipment on the periphery.
For this reason, attempts have been made for a long time in the field of high-frequency technology to integrate a galvanic isolation on the high-frequency side, since, here, mostly only the signal line and reference line need to be galvanically isolated, a matter which can be implemented by means of cost-favorable, planar, waveguide technology.
In WO 03/063190 A2, a simple galvanic isolation via an HF plug connection is described for complying with the ignition protection category “Intrinsically Safe”. The coaxial plug system is composed of a socket and a plug containing a separating layer for galvanic isolation. The plug can also be replaced by a semi-rigid cable. This implementation of the requirements of the ignition protection category “Intrinsic Safety” has the disadvantage that the manufacture of the galvanically isolated, plug connection is very difficulty embodied and expensive. Furthermore, the signals in these junctions of the plug connection are very strongly reflected back, respectively damped, by the geometrical jumps of the line structures in the plug system. For high frequencies, for example over 20 GHz, the galvanic isolation of the inner conductor is, for reasons of the geometry of the plug connection, no longer implementable by a simple coupling via coupling regions of the plug connection, so that the inner conductor must then be galvanically isolated by a further component, especially a capacitor.
In DE 199 58 560 A1, a form of galvanic isolation of high-frequency signals is disclosed, based on use of a slot line. In this document, two slot lines are arranged in parallel, one above the other, so that the electromagnetic radiation issued from the one slot line is coupled into the other slot line. In this way, a galvanically isolated connection is achieved between the two slot lines. The adapting of the slot lines through a dividing layer is implemented via a microstrip line.
Disadvantageous in this form of manufacture is the complicated, multilayered construction of the coupling structure, which requires a plurality of manufacturing process steps. Moreover, the slot, and microstrip, line structure has, compared with a coplanar line, a much greater dispersion, i.e. a much greater dependency of the propagation velocity of the electromagnetic waves relative to wavelength, and frequency.
In EP 0882 995 A1 EP 0882 955 A1, several types of galvanic isolation of a coplanar, conductive-trace structure are disclosed. In the case of coplanar conductor technology, three separate planar waveguides are applied alongside one another on a substrate, with the central, planar waveguide carrying the signal and the two ther planar waveguides bordering the central one and forming the shielding line. Several options for galvanic isolation of the planar waveguides are disclosed in this docuent:                The planar waveguides are separated, and a capacitor is placed in the location of separation; and        the lines are again simply separated, with one line segment being placed on the oppositely lying side of the substrate and segments of the lines spatially overlap—the signal is capacitively coupled through these overlapping segments.        
A disadvantage of this galvanic isolation apparatus is that these measures exhibit good coupling properties only up to a frequency region of several GHz. They are no longer sufficient for higher frequencies above about 6 to 10 GHz. The reason is that the quality of the capacitive coupling of the signal through a substrate of thickness >1 mm is too small.